Method of making a gas sensor

ABSTRACT

A method for preparing a gas sensor includes forming a mixture containing an aqueous buffer solution including a dissolved dye, a polymeric precursor of a cross-linked polymeric material, and a cross-linking agent and reacting the polymeric precursor and the cross-linking agent in the mixture to form a gas sensor including micro-compartments of the aqueous buffer solution dispersed in the cross-linked polymeric material.

This application is a division of application Ser. No. 917,912, filedOct. 10, 1986.

BACKGROUND OF THE INVENTION

This invention is directed to an improved gas sensor for use in sensingbiologically important gases in vivo. The gas sensor includes anemulsoid of an aqueous dye containing buffer solution in a gas permeablecross-linked polymer.

In many situations, it is extremely important to determine the partialpressure of a gas in a fluid. One such situation is the determination ofblood gas in warm blooded animals as for instance the concentration ofblood gas in a human patient's blood during the performance of medicalprocedure on that patient.

Lubbers et al, U.S. Pat. Res. 31,879, describes an apparatus for makingsuch a determination. The apparatus of Lubbers et al utilizes light of aparticular frequency band width which is directed onto a sample cell(which Lubbers refers to as an optode). A biological fluid of interestis positioned in association with the cell for the determination of thepartial pressure of the gas therein.

Heitzmann, U.S. Pat. No. 4,557,900, is also directed to gas sensingdevices. In Heitzmann, dye particles are taken up in a hydrophilic fluidand then are absorded or absorbed on carrier beads of particles. Thesebeads or particles are then taken up in a matrix of a hydrophobicmaterial. Thus for instance Heitzmann forms a solution ofbeta-methylum-belliferone in sodium bicarbonate and absorbs thissolution into the voids of polyacrylamide particles. These particles areof the order of 35 microns in size. The particles are then incorporatedinto a polymerized disk of polymeric material. The disk, which may be ofa size of approximately 100 microns thick by 3 mm in diameter, isassociated with a fiber optic bundle in a cassette. The cassette is thenutilized for sensing blood gases.

The devices of both Lubbers et al and Heitzmann incorporated very usefulblood gas measurement techniques. Both Lubbers et al and Heitzmann takeadvantage of fluorescent or absorption properties of a dye uponinteraction either directly or indirectly with the gas of interest. Theinteraction of the dye with the gas is then measuredspectrophotometrically. This technique has many basic merits and, infact, similar techniques are utilized for this invention. In bothLubbers et al and Heitzmann the before mentioned optical indicator dyeis used to indirectly measure carbon dioxide through an acid baseinteraction of the gas and the dye.

BRIEF DESCRIPTION OF THE INVENTION

This invention eliminates the hydrophilic beads or carrier particles ofthe prior art and facilitates the production of new and improved gassensors of such a size so as to be capable of being introduced directlyinto the body of a patient as for instance by intravenous, intraarterialor interstitial introduction. The gas sensors of the invention arestable, reproducible and are tolerant of production variables withoutdetracting from the inherent properties of the gas sensors.

This can be advantageously accomplished in a gas sensor which comprisesa aqueous first phase including a dye and a second phase. The firstphase comprises an aqueous buffer solvent and a solute, the soluteincluding the dye, the dye being soluble in the aqueous buffer solvent.The second phase comprises a cross-linked polymeric material which isgas permeable, light permeable and essentially aqueous impermeable. Thefirst and second phases are formed into a permeable emulsoid ofsuspended dispersed micro-compartments of the aqueous first phase in thecross-linked polymeric second phase wherein the micro-compartments ofthe aqueous first phase are essentially smaller than 5 microns.

In an illustrative embodiment of the invention, the dye is a pHsensitive dye, the aqueous buffer solvent is a physiological pH rangebuffer solution as for instance a bicarbonate ion based buffer solution.In this illustrative embodiment the polymeric material is a siliconematerial as for instance a siloxane material which is carbon dioxidepermeable. More specifically the material is polydimethysiloxane. Thedye in the illustrative embodiment is hydroxypyrene trisulfonic acid andthe micro-compartments of the aqueous first phase are essentiallysmaller than 2 microns.

In an illustrative embodiment the permanent emulsoid is formed as athree dimensional structure having essentially orthogonal axes andwherein the outside dimension of the structure as measured along any ofthe axes is not greater than 125 microns.

In an illustrative embodiment, the aqueous first phase can furtherinclude at least one emulsification enhancement agent as for instance anemulsification enhancement agent chosen from the group consisting ofwater soluble dextran and polyvinylalcohol. In these illustrativeembodiments, the water soluble dextran would have a molecular weight ofapproximately 500,000 molecular weight units. Further an osmoregulatoryagent and a bacteriostatic agent can be added.

A further advantageous process of preparing a gas sensor comprisesdissolving a quantity of a dye in a quantity of aqueous buffer solutionfollowed by vigorously mixing the buffer solution with a quantity of apolymeric precursor of a cross-linked polymeric material so as to forman emulsion of the buffer solution and the polymeric precursor. Then aquantity of a cross-linking agent is added to the emulsion. A catalystwould be added either to the polymeric precursor or with thecross-linking agent. The catalyzed emulsion is formed into a shape andthe shaped catalyzed emulsion cured by aging to form a permanentemulsoid of micro-compartments of the dye containing aqueous buffersolution in the cross-linked polymeric material.

The above process can be augmented by adding a quantity of anemulsification enhancement agent to the solution of the dye in thebuffer so as to form a solution of the dye and the emulsificationenhancement agent in the buffer.

In an illustrative embodiment of the process, the dye is present in theaqueous phase in a concentration of about 2 to about 50 millimolar andthe buffer is present in the aqueous phase in a concentration of fromabout 2 to about 50 millimolar. The aqueous phase solution of the dye inthe buffer is added to the polymeric precursor in a quantity of about 1gram of the aqueous solution to about 4 grams of the aqueous solutionper 10 grams of the polymeric precursor. The cross-linking agent isadded in a quantity of about 0.5 grams of the cross-linking agent toabout 2 grams of the cross-linking agent per 10 grams of the polymericprecursor and the catalyst is present in trace amounts.

If the emulsification enhancement agent is used, the emulsificationenhancement agent is added to the buffer solution in a amount of about10% to about 30% by weight of the emulsification enhancement agent perthe weight of the buffer solution.

BRIEF DESCRIPTION OF THE DRAWING

This invention will be better understood when taken in conjunction withthe drawing wherein:

FIG. 1 is a elevational view in section of a droplet of materialutilized in the preparation of a gas sensor of the invention; and

FIG. 2 is an elevation view in partial section of a gas sensor of theinvention.

This invention utilizes certain principles and/or concepts as are setforthin the claims appended to this specification. Those skilled in thegas sensing arts to which this invention pertains will realize thatthese principles and/or concepts are capable of being illustrated in avariety of embodiments which may differ from the exact embodimentsutilized for illustrative purposes in this specification. For thesereasons, the invention described in this specification is not to beconstrued as being limited to only the illustrative embodiments but isonly to be construed in view of the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

This invention is directed to a gas sensor which can be utilized with afiber optical cable, i.e. a single optical fiber or a bundle of thesame. The fiber optic cable is associated with appropriate optical andelectronic devices for imposing an optical signal on the fiber and forreading a return optical signal from the gas sensor. A plurality oftechniques for transmitting and reading appropriate optical signals canbeutilized with the gas sensors of the invention. Since the optics andelectronics for gas sensing do not form a part of this invention, forbrevity of this specification, these will not be reviewed in detail,references being made to the above referenced patents to Lubbers et alandHeitzmann. For these reasons the entire disclosures of U. S. Pat. Re.31,879 to Lubbers et al and U. S. Pat. No. 4,557,900 to Heitzmann areherein incorporated by reference.

As with the above referenced patents to Lubbers et al and Heitzmann, adye or optical indicator is utilized for sensing a gas of interest. Thedye can be one which acts with the gas of interest either by directlyinteracting with the gas or by indirectly acting with the gas, as forexample by sensing a pH change in a medium wherein the pH change iscausedby interaction of the gas of interest with that medium.Interaction of the gas of interest with the dye, either directly orindirectly, can be monitored by any suitable optical technique as forinstance by either fluorescence or by absorption.

A suitable optical fiber for carrying an optical signal to and from agas sensor typically need to be no bigger than 125 microns in diameter.Certain prior known gas sensors are many fold times bigger than such aoptical fibers, as for instance the 3000 micron diameter disk ofHeitzmann. The gas sensor of this invention can be sized so as to be ofthe same dimensional realm as such a typical 125 micron optical fiber.In view of this, it is possible to mount the gas sensor of thisinvention directly on the optical fiber utilizing suitable mountingtechniques. As so mounted, together both the gas sensor and itscombination supporting and signal carrying optical fiber can be inserteddirected into a system of interest as for instance directly into apatient's cardiovascular system, intramuscularly or into other bodyorgans such as the lungs and the like.

In constructing a gas sensor of this invention, we form a solution of asuitable indicator dye in an aqueous buffer. The aqueous phase is thenemulsified with a liquid precursor of a polymeric material. During theemulsification step, the aqueous phase is broken up into very smalldroplet sizes. The polymeric material is chosen such that the aqueousphase is not soluble in either the precursor materials for the polymericmaterial or the polymerized polymeric material. Thus the aqueous phasealways retains its integrity.

By emulsifying the aqueous phase into the polymeric precursor materials,very small discrete "micro-compartments" or cells of the aqueous phasecanbe formed in the polymeric phase. Upon polymerization, thesemicro-compartments are "frozen" or fixed in dispersed positions whichare essentially uniformly scattered throughout the polymeric material.An emulsoid of the aqueous phase is thus formed in the polymeric phase.

Since the aqueous phase is very evenly distributed within the polymericphase, when it is fixed in position in the emulsoid, its concentrationis very evenly distributed throughout the emulsoid. Because theconcentrationof the aqueous phase is uniform through the emulsoid, thesensing characterics of the gas sensor of the invention are also veryuniform.

Contrary to other gas sensors, by using very small emulsion sizedparticles, the surface area of the individual micro-compartments andthus the totality of the micro-compartments of the aqueous phase is verylarge.Gas exchange between the polymeric phase and the aqueous phase isacross the interface between the surface of the aqueous phase and thepolymeric phase. Because the surface area of the aqueous phase which isin contact with the surface area of the polymeric phase is very large,for the gas sensors of this invention, gas exchange to the sensingaqueous phase is fast and is uniformly sensitive to the gasconcentration within the polymeric phase.

By emulsifying the aqueous phase into the polymeric phase, there is nonecessity for a further supporting phase as in other prior art devices.Greater miniaturization is thus possible by elimination of the supportphase particles which are of a size domain much greater than the sizedomain of the individual aqueous phase micro=compartments of thisinvention. With support phase particles eliminated, each of the dropletsis in contact over substantially its full surface area with thepolymeric phase and is solely supported by the polymeric phase.

At a minimum, the aqueous phase must contain an indicator of the gas ofinterest for which the sensor is being used. Other materials can beincorporated into the aqueous phase micro-compartments. The aqueousphase must however serve as a solvent for these other materials, that isthey must all be in the solute of the aqueous phase. Depending on thegas of interest, these other materials would be chosen to contribute tothe operating characterics of the gas sensor. As for instance, theseadditional materials can be added to promote the emulsification of theaqueous phase into the polymeric phase. Further they can be added tolowerthe vapor pressure of the aqueous phase in the polymeric phase soas to retard the evaporation of the aqueous phase during formation ofthe gas sensor of interest.

Aside from materials which contribute to the physical formation of theemulsoid of the aqueous phase in the polymeric phase, further additivescan be added to the aqueous phase for enhancement of the storage and/oroperating characterics of the gas sensor as for instance bacteriostaticand/or osmoregulatory agents. These would also be chosen to be part ofthesolute of the aqueous first phase.

The polymeric phase is chosen as a carrier for the aqueous phase and tomaintain the individual micro-compartments of the aqueous phase in theirdispersed form. The polymeric phase must be permeable to the gas ofinterest. It must also be permeable to the wavelength or wavelengths oflight utilized in the measurement of the gas of interest. Further sinceitis necessary to maintain the aqueous phase isolated from the carrierfluid of the gas of interest, the polymeric phase must be impermeable toliquid water. In order to isolate the indicator and/or any otheringredients in the aqueous phase, the polymeric phase must also beimpermeable to ionic species. As for instance if the aqueous phasecontains a buffer, it is important to maintain the ionic concentrationof the buffer ions constant and not to dilute out or to increase theconcentration of these desired buffer ions.

Fillers can be added to the polymeric phase. However it is importantthat if such fillers are added, they contribute only to desired orenhanced properties of the polymeric phase and do not interfere with ordetract from the aqueous phase or the emulsification of the aqueousphase into thepolymeric phase. Any such filler in the polymeric phasewould not be of a size or nature to serve as carrier particles for theaqueous phase. They would be added to enhance the structuralcharacteristics of the polymeric phase. Such enhancements might be madeto reinforce the polymeric phase orto stabilize the polymeric phase.Depending of the polymeric phase material, catalyst molecules orparticles might also remain in the polymeric phase after completion ofthe polymerization, as for instance metallic catalyst particles.

A particular gas of interest for the gas sensor of this invention iscarbondioxide. For sensing carbon dioxide a pH sensitive dye would besolubilizedin the aqueous phase. Gas exchange through the polymericphase and into theaqueous phase solubilizes the carbon dioxide gas inthe aqueous phase as carbonic acid which interacts with the buffer ions.The dye chosen is one which is responsive to the concentrations of theionic species of the carbonic acid in the aqueous phase, i.e. anacid-base responsive dye.

Preferred for use in sensing carbon dioxide is a bicarbonate ion basedbuffer aqueous phase. Such a buffer can be chosen so as to have a bufferrange compatible with the response range of the dye. Such a range might,for instance, mimick the physiological pH range of blood. Suitable forthepreparation of such a bicarbonate ion buffer would be sodiumbicarbonate, sodium carbonate and sodium hydroxide or other suitablebuffer agents. Formeasuring blood carbon dioxide with hydroxypyrenetrisulfonic acid, a pH range of pH 7.0 to pH 8.0 is the most desirable.

In choosing a dye for measuring carbon dioxide in blood, considerationis given to matching the pKa of the dye to the pH range of the bufferinducedby physical CO₂ levels. In constructing a gas sensor of thisinventionfor use in sensing carbon dioxide gas in blood, we have foundthat hydroxypyrene 3,6,8-trisulfonic acid has characterics which aresuperior to beta-methyumbelliferone. Although beta-methylumbelliferonecan also be used. Hydroxypyrene 3,6,8-trisulfonic acid, herein afterreferred to as HPTS, which is a known fluorescence dye for carbondioxide, has a larger "Stokes shift" than does the umbelliferonecompound. For use in fluorescences spectroscopy, this separates theexcitation light from the emission light which improves the measurementof the emission light for improved gas sensor performance. Thehydroxypyrene trisulfonic acid can beused as a free acid or as one ofits salts as for instance an alkali or alkali earth salt.

In addition to the above mentioned HPTS and umbelliferone compound,other fluorescent dyes such as fluorescein could be used. Alsoabsorption dyes such as chlorophenol red, bromo cresol purple,nitrophenol, bromo thymol blue, pinachrome and phenol red could be used.

Because of their high gas permeability and water impermeability,silicone polymers are preferred. For use in forming a carbon dioxide gassensor, polydimethysiloxane is preferred. A particular useful commercialpolydimethysiloxane is Petrarch PE1055. It is used in conjunction with across-linking agent Petrarch PE1055 cross-linker or Petrarch PE123cross-linker. The PE1055 has a platinum catalyst incorporated into thepolymeric precursor which is retained in the polymerized polymer. Itspresence in the finished gas sensor does not detract from thecharactericsof the gas sensor. With other polymers, volatile or shortlived catalysts such as a free radical catalyst could be used.

The silicone polymeric phase can be prepared via two polymerizationreactions. Aside from the above described addition type polymerization,the silicone polymeric phase can also be prepared via condensationpolymerization reactions using silanol terminated silicones cross-linkedwith alkoxyl silanes using catyalsts such as tin derivatives.

The above noted PE1055 siloxane precursor also contains a fumed silicafiller. This filler enhances the stability of the aqueous dispersion andthe tear strength of the final polymeric product.

During manufacturing of the carbon dioxide sensor of the invention, theabove noted HPTS is taken up in an appropriate buffer solution. Anappropriate bacteriostatic agent such as thimerosal is added as issodium chloride as an osmoregulatory agent.

Certain properties of the emulsion between the aqueous phase and thepolymeric precursor can be enhanced by adding additional agents hereinidentified by the terminology "emulsification enhancement agents". Theseemulsification enhancement agents enhance certain manufacturingpropertiessuch as shelf life of the gas sensor intermediates bystabilizing the emulsion and retarding dehydration of the aqueous phase.In general these emulsification enhancement agents are hydrophilic macromolecules.

By retarding the dehydration of the aqueous phase and break down of theemulsion of the aqueous phase and the polymeric precursor, it is notmandatory to immediately polymerize the aqueous phase-polymericprecursor emulsion into the emulsoid gas sensor of the invention. Withthe addition of the emulsification enhancement agents, the emulsion ofthe aqueous phase and polymeric precursor is stable and can set asidefor formation into the emulsoid gas sensor of the invention at a latertime. This reduces adhering to a tight manufacturing schedule andreduces or preventsthe generation of manufacturing "scrap materials"which are economically wasteful.

Suitable for use as the above mentioned hydrophilic macro molecules usedasthe emulsification enhancement agents would be agents such as watersolubledextran. Also considered might be polyvinylalcohol. For use inthis invention, water soluble dextran as opposed to cross-linked dextranwould be used to insure its water solubility. One such water solubledextran is dextran of a molecular weight of about 500,000. In any event,if added theemulsification enhancement agent or any other agents such asthe before mentioned bacteriostatic or osmoregulatory agents would bewater soluble and would form part of the solute of the aqueous bufferphase. Since in the finished gas sensor of this invention, these soluteagents always remain in solution even in the finished gas sensor, theydo not detract from the above noted properties of the gas sensor of thisinvention.

Generally the concentration of the dye in the aqueous phase would befrom about 2 millimolar to about 50 millimolar with about a 10millimolar solution being typically used. Generally the concentration ofthe buffer in the aqueous phase would be from about 2 millimolar toabout 50 millimolar with about a 20 millimolar solution typically beingused. If used, the emulsification enhancement agent would be present atfrom about 10% to about 30% by weight per weight of the water of theaqueous phase. Typically about a 25% by weight of the emulsificationenhancement agent isused. From about 1 grams to about 4 grams of theaqueous solution would be added to about 10 grams of the polymericprecursor. Typically about 2 grams of the aqueous phase per 10 grams ofthe polymeric precursor is used. The cross-linking agent would be addedfrom about 5% to about 20% byweight of the polymeric precursor withapproximately 10% by weight with respect to the weight of the polymericprecursor typically being used.

In an illustrative embodiment of the invention the dye is present in aquantity of about 0.01 grams per 2 mls. of said buffer solution, the dyein the buffer solution is added to the polymeric precursor in a quantityof about 2 mls. of the dye in the buffer solution to about 10 grams ofthepolymeric precursor. The cross-linking agent is added in a quantityof about 1 gram of the cross-linking agent per 10 grams of the polymericprecursor and the catalyst is present in trace amounts.

When the mixture of the aqueous phase and the polymeric precursor isemulsified, a suitable homogenizer such as a Virtis 23 homogenizer isused. The emulsification enhancement agent contributes to stability oftheemulsion such that it has an increased shelf life. When it is desiredto form the gas sensor of the invention, the cross-linker is added, asis thecatalyst if it is not already present in the polymeric precursor.These aregently stirred into the emulsion and the resulting mixture thenshaped and cured by aging. A very simple gas sensor can be formed bysimply depositing a drop of the mixture of the emulsion and thecross-linking agent on to the end of a fiber optic fiber and allowing itto cure into anemulsoid directly on the end of the fiber.

Following emulsification, the aqueous phase is present in the polymericprecursor in micro-compartments which are generally essentially smallerthan 5 microns. Typically a production gas sensor of the invention willhave micro compartments of the aqueous phase in the polymeric phasewherein the majority of the population of the compartments will be onthe order of 2 microns. It is of course realized that the particles willactually be in a statistical range of particle sizes, some slightlylargerthan the above noted sizes, some slightly smaller, depending onthe emulsification procedure and apparatus.

Seen in FIG. 1 is a drop 10 of the emulsion of the aqueous phase in thepolymeric precursor. As is evident the micro-compartments 12 aredispersedin a uniform manner through the drop 10 of the emulsion.

For formation of a very simple gas sensor 14 of this invention, in FIG.2, a drop of the above mixture is placed on the distal end 15 of anoptical fiber 16. As the mixture of the cross-linking agent and thepolymeric precursor having the aqueous phase as a emulsion therein,ages, it cures into an emulsoid 18 of the micro-compartments 20 of theaqueous phase in the polymeric material of carrier body 22. If desired,the emulsoid 18 canbe retained on the end of the fiber 16 using asuitable sleeve 24. The sleeve 24 can be constructed from a suitablematerial such as TEFLON (polytetrafluoroethylene) or the like. Furtherto avoid light intensity changes caused by factors other than thechanges in patial pressure of thegas sensed, an overcoat 26 can be addedas a layer over the exposed portions of the emulsoid 18. For use with afluorescent dye, the overcoat 26 is chosen to be opaque to theexcitation light wavelength, λ_(ex) and to the emission light wavelengthλ_(em) both of which are transmitted along the same single optical fiber16. A suitable material for the overcoat 26 would be celluloseimpregnated with carbon black.

As is evident in FIG. 2, the size of the gas sensor 14 is dictated onlyby the optical fiber size. The gas sensor 14 thus formed is of asufficientlysmall size so as to be introducible directed into thecardiovascular systemof a patient for direct real time measurement ofthe partial pressure of a blood gas such as carbon dioxide. If the fiberoptic fiber 16 of FIG. 2 istypically about 125 micron in diameter, it isevident that the emulsoid 18 is approximately equal to or less than thissize in each of it orthogonally oriented width, height and depthdimensions. Other constructions of gas sensors are also possibleutilizing the emulsoid of this invention. It of course being realizedthat smaller sensors could be constructed by utilizing a smallerdiameter fiber optic cable.

By using the above noted gas sensor construction in conjunction withHPTS as a carbon dioxide sensitive dye, determination time of the gas ofinterest is made in a time period of approximately one minute. This gassensor can be autoclaved to sterilize it without detracting from ordegrading it performance and during its use it is essentiallytemperature stable.

The following embodiment is offered as an illustration for thepreparation of a gas sensor of the invention. 0.1048 grams of HPTS,0.0106 grams of sodium bicarbonate, 0.1284 grams of sodium chloride6.666 grams of 500,000mol. wt. dextran and 0.02 grams of thimerasol weredissolved in 20 grams ofwater. 2 mls of the above aqueous solution wasmixed with 10 grams of Petarach PE1055, which contained a trace amountof a platinum catalyst, ina Virtis 23 homogenizer for 30 seconds atmoderate speed then for 3 minutesat high speed with a cooling timeinbetween of 30 to 45 seconds to form an emulsion. 1.0 grams of PetarachPE1055 cross-linker was added and stirred into the other ingredients. Adrop of the final emulsion was then placed on the end of a 125 microndiameter fiber optic cable to form a carbon dioxide gas sensor. Theagueous phase micro compartments in this sensor were on the order ofapproximately 2 microns in diameter. The final sensoritself, haddimension approximately equal to that of the fiber optic cable.In eachof the three X, Y, and Z orthogonal axes, the sensor was less than 125microns in size.

We claim:
 1. A method for preparing a gas sensor comprising:forming amixture containing an aqueous buffer solution including a dissolved dye,a polymeric precursor of a cross-linked polymeric material, and across-linking agent; and reacting said polymeric precursor and saidcross-linking agent in said mixture to form a gas sensor comprisingmicro-compartments of said aqueous buffer solution dispersed in saidcross-linked polymeric material.
 2. The method of claim 1 wherein saidmixture is an emulsion.
 3. The method of claim 1 wherein said aqueousbuffer solution further includes an emulsification enhancement agent. 4.The method of claim 1 wherein said cross-linked polymeric material isgas and light permeable, and substantially ion and aqueous impermeable.5. The method of claim 1 wherein said dye is present in said aqueousbuffer solution in a concentration in the range of about 2 millimolar toabout 50 millimolar.
 6. The method of claim 1 wherein said aqueousbuffer solution includes a buffer present in a concentration in therange of about 2 millimolar to about 50 millimolar.
 7. The method ofclaim 1 wherein said mixture includes about 1 gram to about 4 grams ofsaid aqueous buffer solution per 10 grams of said polymeric precursor.8. The method of claim 1 wherein said mixture includes about 0.5 gramsto about 2 grams of said cross-linking agent per 10 grams of saidpolymeric precursor.
 9. The method of claim 1 wherein said mixturefurther includes a catalyst effective to promote said reaction.
 10. Themethod of claim 3 herein said emulsification enhancement agent ispresent in an amount in the range of about 10% to about 30% by weight ofsaid aqueous buffer solution.
 11. The method of claim 1 wherein said dyeis a pH sensitive dye.
 12. The method of claim 1 wherein said aqueousbuffer solution is a bicarbonate ion based buffer solution.
 13. Themethod of claim 1 wherein said gas sensor is a carbon dioxide sensor.14. The method of claim 1 wherein said dye is hydroxypyrene 3, 6,8-trisulfonic acid or a salt thereof.
 15. The method of claim 1 whereinsaid micro-compartments are smaller than 5 microns.
 16. The method ofclaim 1 wherein said dye is present in said mixture in an amount ofabout 0.01 grams per 2 mls. of said aqueous buffer solution, saidaqueous buffer solution is present in said mixture in an amount of about2 mls. per 10 grams of said polymeric precursor, and said cross-linkingagent is present in said mixture in an amount of about 1 gram per 10grams of said polymeric precursor.
 17. The method of claim 16 whereinsaid aqueous buffer solution further includes about 0.66 grams of anemulsification enhancement agent per 2 mls of said aqueous buffersolution.
 18. The method of claim 1 which further comprises placing saidmixture on the end of an optic fiber prior to said reacting step.
 19. Amethod for preparing a carbon dioxide sensor comprising:forming amixture containing an aqueous buffer solution including a dissolved pHsensitive dye, a polymeric precursor of a cross-linked, light and gaspermeable, substantially ion and aqueous impermeable polymeric material,a cross-linking agent and a catalyst; and reacting said polymericprecursor and said cross-linking agent in said mixture to form a carbondioxide sensor comprising micro-compartments of said aqueous buffersolution dispersed in said cross-linked polymeric material.
 20. Themethod of claim 19 which further comprises placing said mixture on theend of an optical fiber prior to said reacting step.